1. Field of the Invention
This invention relates to control of a transport refrigeration unit that has at least heating and cooling capabilities and an unloadable compressor.
2. Description of the Prior Art
In many typical transport refrigeration units a throttling valve is provided in the suction line to which the refrigerant returns to the compressor. This valve, sometimes referred to as a suction hold-back valve, regulates the amount of refrigerant returning to the compressed compressor and by controlling this amount the load can be controlled to limit the horsepower required, particularly in a heating or defrost mode, to prevent an overload of the driving engine. While it would be desirable to eliminate the throttling valve, both because of its initial cost and because it adds some restriction at all times in the line, some means is still required for avoiding overload with higher suction pressures. Of course the typical high pressure cutout will function to stop the operation with an excessive head pressure when heating, but this results in cycling of the unit.
U.S. Pat. No. 3,010,289 discloses an arrangement for a transport refrigeration unit control which includes a high pressure switch which can function to unload the compressor when a predetermined abnormally high head pressure in the cylinders of the compressor is sensed. This partial unloading will occur with a high head pressure while the refrigeration system is operating either on a heating or a cooling cycle. This arrangement is considered disadvantageous since in a cooling mode significantly higher discharge and suction pressures can be accommodated without an overload problem than in a heating or defrost mode.
It is the aim of this invention to provide a transport refrigeration unit and control arrangement in which the throttling valve is eliminated, and in which the disadvantage of the arrangement of the noted patent is also avoided.